1. Field of the Invention
The present invention generally relates to the field of microwave frequency oscillators, and more specifically to a variable frequency oscillator which includes a digital phase shifter as a tuning element and can be advantageously utilized in a monolithic microwave integrated circuit (MMIC).
2. Description of the Related Art
Analog voltage-controlled oscillators (VCOs) are widely used as sources of signals at microwave frequencies. The resonant circuits in these oscillators generally include a variable capacitance diode or varactor as a tuning element. The capacitance of the varactor and thereby the resonant frequency of the oscillator vary in accordance with the value of an analog control voltage applied to the varactor.
An example of a prior art microwave oscillator which is conventionally used in MMIC applications is illustrated in FIG. 1 and generally designated as 10. A resonant circuit or resonator 12 is connected in series with a negative resistance element in the form of a field effect transistor (FET) Q1. More specifically, the resonator 12 has one terminal 14 connected through a gate matching network 16 to the gate of the FET Q1 and a second terminal 18 connected to ground, and bias is supplied to D1 through a radio frequency (RF) blocking choke L1 and an RF bypass capacitor C1.
The resonator 12 includes a transmission line segment 20 which is connected in series with an inductor L2 between the terminals 14 and 18. The segment 20 represents the length of transmission line required to interconnect the resonator 12 with the other elements of the oscillator 10, and can be inductive or capacitive depending on its length and the frequency of oscillation. A DC blocking capacitor C2 is connected in series with a variable capacitance diode or varactor D1 in parallel with the inductor L2.
The FET Q1 is connected in a common-source configuration with its source connected to ground through a source matching network 22. The drain of the FET Q1 is connected through an output matching network 24 to an output terminal 26. Although not illustrated, the FET Q1 can also be connected in common-drain and common-gate configurations.
The oscillator 10 is known in the art as a "negative resistance" type amplifier. The inductor L2 and varactor D1 constitute the main elements of a parallel resonant circuit having a resonant frequency determined by the inductance of the inductor L2 and the capacitance of the varactor D1. Oscillation is initiated and sustained in the resonant circuit 12 and an oscillating signal coupled to the output terminal 26 due to the negative resistance of the FET Q1. The general principles of negative resistance amplifiers are set forth in a textbook entitled "GaAs MESFET Circuit Design", edited by R. Soares, published by Artech House, 1988. pp. 347-348.
The capacitance of the varactor D1 is variably set and the frequency of oscillation of the oscillator 10 is correspondingly variably set by applying an analog DC control or tuning voltage Vc to the terminal 18. A controller 28, which may be a conventional microprocessor, includes a central processing unit 30, a read-write or random-access memory (RAM) 32 and a program memory 34 which can be either read-write or read-only (ROM). A look-up table which contains values of the tuning voltage Vc and the oscillator frequency which results from application of the corresponding tuning voltage Vc to the varactor D1 are stored in a table memory 36. Although not illustrated, the table may alternatively be stored in the ROM 34 or RAM 32.
To select a desired frequency of oscillation of the oscillator 10, a command is entered into the CPU 30 from a keyboard or the like (not shown) designating the desired frequency. The CPU 30 accesses the table ROM 36 with the designated frequency and retrieves the value of tuning voltage Vc which corresponds to the accessed frequency. The CPU 30 then applies a digital code corresponding to the retrieved tuning voltage Vc to a digital-to-analog converter 38 which converts the code into the analog tuning voltage Vc and applies the voltage Vc to the varactor D1 via the terminal 18.
FIG. 2 illustrates a similar prior art microwave oscillator 10' in which a resonator 12' and negative resistance element in the form of an NPN type bipolar transistor Q2 are connected in a parallel. Like elements are designated by the same reference numerals as in FIG. 1, and corresponding but modified elements are designated by the same reference numerals primed.
The transistor Q2 is connected in a common-emitter amplifier configuration, although common-collector and common-base arrangements are also conventionally used. The resonator 12' is connected in the feedback path of the amplifier, between the collector and base of the transistor Q2. The varactor D1 and an inductor L2' constitute the main elements of a series resonant circuit, and are connected in series with a transmission line segment 20' between terminals 14' and 18'. Further illustrated is a DC blocking choke L3 which is connected from the junction of the varactor D1 and transmission line segment 20' to ground.
The operation of the oscillator 10' is similar to that of the oscillator 10, except that oscillation is initiated and sustained in the resonant circuit 12' and an oscillating signal coupled to the output terminal 26 due to positive feedback from the collector to the base of the transistor Q2.
Where the varactor D1 is a conventional regular-abrupt varactor diode, the frequency tuning range of the oscillators 10 and 10' is limited to approximately 5-10% due to limited capacitance range. In addition, the control voltage Vc is generally limited to a relatively small range, on the order of 10 volts. The tuning range can be increased by utilizing a hyper-abrupt varactor diode which has a larger range of capacitance, on the order of 40-50%. However, the phase noise of a hyper-abrupt varactor diode is much higher due to lower quality factor (Q).
Advanced applications such as Near Obstacle Detection Systems (NODS) for automobiles and other vehicles utilize Doppler radars including MMIC transceivers. The oscillators for the transceivers must have a precise and stable center frequency, and must be capable of highly linear monotonic frequency stepping under digital control. These goals are difficult to attain with an analog VCO. Due to practical processing tolerances, it is extremely difficult to accurately set the center frequency of the oscillator at a design value. External tuning using laser scribing or the like is required for setting the center frequency. This is a complex and expensive processing step, and is impractical where large quantities of products must be manufactured at low unit cost. Production yield losses due to frequency inaccuracy can be very high.
Varactor diodes also have highly non-linear voltage/capacitance characteristics, making it extremely difficult to provide linear frequency stepping. The table memory 36 is therefore an essential part of the oscillator 10 or 10', and generation of the table in view of the non-linearities involved is difficult on a commercial production basis.